Slurry hydrocarbon synthesis (HCS) processes are known. In a slurry HCS process a synthesis gas (syngas) containing a mixture of H2 and CO is bubbled upward through a slurry in a reactor comprised of hydrocarbon reaction products having dispersed therein a Fischer-Tropsch type hydrocarbon synthesis catalyst. Reactors that contain such a three phase slurry are sometimes referred to as “bubble columns”, as is disclosed in U.S. Pat. No. 5,348,982, which is incorporated herein by reference. Irrespective of whether the slurry reactor is operated as a dispersed or slumped bed, the mixing conditions in the slurry will typically be somewhere between the two theoretical conditions of plug flow and back mixed. The catalyst particles are typically kept dispersed and suspended in the liquid by the lifting action of the syngas bubbling up through the slurry and by hydraulic means. Slurry Fischer-Tropsch reactors produce a vapor phase and a higher molecular weight liquid product stream.
Because of the formation of liquid products (commonly called waxes in this context), it is necessary to maintain the slurry at a constant level by continuously or intermittently removing liquid products from the reactor. One problem with the removal of liquids, however, is that catalyst particles are dispersed in the liquid and must be separated from the said liquid and returned to the reactor slurry in order to maintain a constant inventory of catalyst in the reactor. Several means have been proposed for separating the catalyst from the liquid products, e.g., centrifuges, sintered metal filters, cross-flow filters, magnetic separators, gravitational settling, etc.
Filtration is one of the catalyst-liquid separation methods used with Fischer-Tropsch reactors. Filtration techniques are characterized by solid-liquid separation systems that remove liquid products from a slurry by drawing the fluid across a filter medium. The filter medium may be simply a filter substrate or may be composed of a filter cake disposed on a filter substrate, such that the filter cake forms a primary filter. A filter cake is formed as solid particles are deposited on the filter substrate creating a permeable barrier between the slurry and the substrate. The thickness and permeability of the filter cake is critical to the efficient operation of the filtration system.
In a commercial slurry bubble column reactor, the hydrodynamic conditions inside the reactor, coupled with the desired long lifetime of the catalytic material, typically results in catalyst attrition. As the catalyst breaks down over time, sub-particles of various sizes are created, including very small particles known as “fines”, some of which may even be sub-micron in size. The presence of fines in the reactor tends to greatly reduce the effectiveness of the catalyst-liquid separation system.
In a slurry reactor, the action of the gas rising through the liquid and solid phases results in agitation and movement of those phases. This agitation has a beneficial effect on the performance of filters that may be immersed in the slurry reactor. The agitation of the slurry generally significantly reduces the accumulation of solids on the surface of the filter due to the drag force of the agitated liquid on the solids at the filter surface. It is known that this drag force is a function of the particle size, with smaller particles experiencing less drag than larger particles. Hence, while it may be possible to operate a filter in a slurry bubble column with large catalyst particles for extended times, finer particles are more easily collected on the filter surface and can contribute to fouling of that filter. Thus, in a catalyst-liquid separation system utilizing filtration, cycle time between backwashing operations, as well as filter life, may be greatly reduced because the fines tend to reduce the permeability and flux of the filter system. The use of centrifuges or gravitational settlers is not practical for removing fines from the catalyst slurry because the fine particles are low in concentration relative to the catalyst. Magnetic separation is similarly impractical for removing fines from the slurry. Thus the performance of catalyst-liquid separation systems has heretofore been undesirably dependent upon the age of the catalyst. For example, when the catalyst is new the catalyst-liquid separation system operates at a very high rate, but the rate substantially decreases as the catalyst ages and attrition causes fines concentration to increase.
It is also undesirable to have particulates present in the liquid product from the HCS reactor. The products from an HCS reactor may be subject to additional processing in systems downstream of the HCS reactor that may have stringent specifications on the concentration of particulates. In addition, final products, e.g., lube oils or diesel, may also have stringent specifications on particulates content.
Thus, there remains a need in the art for methods and apparatus to maintain the effectiveness of a catalyst-liquid separation system independent of the age or degree of attrition of the catalyst. Therefore, the embodiments of the present invention are directed to methods and apparatus for removing catalyst fines from a slurry that seek to overcome the limitations of the prior art.